Discovery of an efficient artificial catalyst for the sunlight-driven splitting of water into dioxygen and dihydrogen is a major goal of renewable energy research. We describe a solution-phase reaction scheme that leads to the stoichiometric liberation of dihydrogen and dioxygen in consecutive thermal- and light-driven steps mediated by mononuclear, well-defined ruthenium complexes. The initial reaction of water at 25 degrees C with a dearomatized ruthenium (II) [Ru(II)] pincer complex yields a monomeric aromatic Ru(II) hydrido-hydroxo complex that, on further reaction with water at 100 degrees C, releases H2 and forms a cis dihydroxo complex. Irradiation of this complex in the 320-to-420-nanometer range liberates oxygen and regenerates the starting hydrido-hydroxo Ru(II) complex, probably by elimination of hydrogen peroxide, which rapidly disproportionates. Isotopic labeling experiments with H2 17O and H2 18O show unequivocally that the process of oxygen-oxygen bond formation is intramolecular, establishing a previously elusive fundamental step toward dioxygen-generating homogeneous catalysis.

The performance of a wide variety of DFT exchange-correlation functionals for a number of late-transition-metal reaction profiles has been considered. Benchmark ab-initio reference data for the prototype reactions Pd + H2, Pd + CH4, Pd + C2H6 (both C-C and C-H activation), and Pd + CH3Cl are presented, while ab-initio data of lesser quality were obtained for the catalytic hydrogenation of acetone and for the low-oxidation-state and high-oxidation-state mechanisms of the Heck reaction. "Kinetics" functionals such as mPW1K, PWB6K, BB1K, and BMK clearly perform more poorly for late-transition-metal reactions than for main-group reactions, as well as compared to general-purpose functionals. There is no single "best functional" for late-transition-metal reactions, but rather a cluster of several functionals (PBE0, B1B95, PW6B95, and TPSS25B95) that perform about equally well; if main-group thermochemical performance is additionally considered, then B1B95 and PW6B95 emerge as the best performers. TPSS25B95 and TPSS33B95 offer attractive performance compromises if weak interactions and main-group barrier heights, respectively, are also important. In the ab-initio calculations, basis set superposition errors (BSSE) can be greatly reduced by ensuring that the metal spd shell has sufficient radial flexibility in the high-exponent range. Optimal HF percentages in hybrid functionals depend on the class of systems considered, increasing from anions to neutrals to cations to main-group barrier heights; transition-metal barrier heights represent an intermediate situation. The use of meta-GGA correlation functionals appears to be quite beneficial.

The new, structurally characterized hydrido carbonyl tetrahydridoborate iron pincer complex [(iPr-PNP)Fe(H)(CO)(η(1)-BH(4))] (1) catalyzes the base-free hydrogenation of ketones to their corresponding alcohols employing only 4.1 atm hydrogen pressure. Turnover numbers up to 1980 at complete conversion of ketone were reached with this system. Treatment of 1 with aniline (as a BH(3) scavenger) resulted in a mixture of trans-[(iPr-PNP)Fe(H)(2)(CO)] (4a) and cis-[(iPr-PNP)Fe(H)(2)(CO)] (4b). The dihydrido complexes 4a and 4b do not react with acetophenone or benzaldehyde, indicating that these complexes are not intermediates in the catalytic reduction of ketones. NMR studies indicate that the tetrahydridoborate ligand in 1 dissociates prior to ketone reduction. DFT calculations show that the mechanism of the iron-catalyzed hydrogenation of ketones involves alcohol-assisted aromatization of the dearomatized complex [(iPr-PNP*)Fe(H)(CO)] (7) to initially give the Fe(0) complex [(iPr-PNP)Fe(CO)] (21) and subsequently [(iPr-PNP)Fe(CO)(EtOH)] (38). Concerted coordination of acetophenone and dual hydrogen-atom transfer from the PNP arm and the coordinated ethanol to, respectively, the carbonyl carbon and oxygen atoms, leads to the dearomatized complex [(iPr-PNP*)Fe(CO)(EtO)(MeCH(OH)Ph)] (32). The catalyst is regenerated by release of 1-phenylethanol, followed by dihydrogen coordination and proton transfer to the coordinated ethoxide ligand.

A new database of transition metal reaction barrier heights-MOBH35-is presented. Benchmark energies (forward and reverse barriers and reaction energy) are calculated using DLPNO-CCSD(T) extrapolated to the complete basis set limit using a Weizmann1-like scheme. Using these benchmark energies, the performance of a wide selection of density functional theory (DFT) exchange-correlation functionals, including the latest from the Truhlar and Head-Gordon groups, is evaluated. It was found, using the def2-TZVPP basis set, that the ωB97M-V (MAD 1.8 kcal/mol), ωB97X-V (MAD 2.1 kcal/mol) and SCAN0 (MAD 2.1 kcal/mol) hybrid functionals are recommended. The double-hybrid functionals PWPB95 (MAD 1.6 kcal/mol) and B2K-PLYP (MAD 1.8 kcal/mol) did perform slightly better but this has to be balanced by their increased computational cost. File list (3) download file view on ChemRxiv TM Barrier Heights-revision 1-v2-for submission.pdf (1.58 MiB) download file view on ChemRxiv MOBH35-SI.pdf (132.27 KiB) download file view on ChemRxiv mobh35.tar.gz (82.99 KiB)

A nonoxidative addition pathway for the activation of NH bonds of ammonia and amines by a Ru(II) complex is reported. The pincer complex 1 cleaves N-H bonds via metal-ligand cooperation involving aromatization of the pincer ligand without a change in the formal oxidation state of the metal. Electron-rich N-H bond substrates lead to reversible activation, while electron-poor substrates result in stable activation products. Isotopic labeling studies using ND(3) as well as density functional theory calculations were used to shed light on the N-H activation mechanism.